When collision energy acts on a vehicle, for example, a vehicle airbag deploys in front of a vehicle occupant sitting in a driver seat or a passenger seat, thereby protecting the vehicle occupant.
Airbags of the type described have a vent hole. The vent hole is a gas-releasing hole through which some of the gas is ejected to the exterior so as to restrict excessive increases in internal pressure when the airbag is deployed. The size of the vent hole is set in advance and does not change. Therefore, the ejection rate or quantity of gas ejected to the exterior from the vent hole cannot be precisely controlled in accordance with the state in which a vehicle occupant collides with the deployed airbag.
From an initial stage in which the airbag begins to deploy in front of the vehicle occupant, the gas inside the airbag begins to be ejected to the exterior through the vent hole. To increase the airbag's performance of protecting the vehicle occupant, it is preferable that the internal pressure of the airbag be maintained over a long period of time. However, the ejection rate and quantity of the gas ejected to the exterior from the vent hole cannot be controlled.
Thus, an improvement can be made in regard to merely having a vent hole in the airbag, in terms of precisely controlling the internal pressure of the airbag. In view whereof, techniques for controlling the internal pressure of an airbag have recently been developed (see Patent Document 1, for example).
The airbag taught in Patent Document 1 has a vent hole cover for closing the vent hole. This vent hole cover comprises a plurality of sewn parts sewn into the airbag. The sewn parts sequentially rupture as the internal pressure increases when the airbag is deployed, and the rupturing concludes at the end of the airbag deployment. As a result, the vent hole cover is removed from the vent hole, and therefore the vent hole is opened. In other words, it is possible to control the timing with which the gas inside the airbag is ejected to the exterior from the vent hole.
However, in the airbag known in Patent Document 1, the size of the vent hole is set in advance and does not change. Therefore, the ejection timing, ejection rate, and quantity of the gas ejected to the exterior from the opened vent hole cannot be precisely controlled in accordance with the state in which the vehicle occupant collides with the deployed airbag. Moreover, in the airbag known in Patent Document 1, since the vent hole cover for closing the vent hole is sewn, controlling the ejection timing, ejection rate, and ejected quantity of the gas in accordance with the state in which the vehicle occupant collides with the deployed airbag inevitably makes the configuration complicated and causes cost to increase.